1. Field of the Invention
The present invention relates to modified pigment products, dispersion compositions of these modified pigment products, and the use of these modified pigment products in several applications, including electrophoretic displays, inks, coatings, and plastics. The present invention also relates to printing plates comprising a substrate and a radiation-absorptive layer, wherein the radiation absorptive layer comprises at least one modified pigment product.
2. Description of the Related Art
Pigments have been used in a variety of applications, such as in inks, coatings, toners, plastics, rubber, films, displays, and the like. Common goals in each of these applications are to provide a pigment which is capable of being substantially and uniformly dispersed, and capable of remaining dispersed in the composition so that color and other properties exhibited by the pigment are uniform throughout the composition or material which incorporates the pigment.
The improvement in the dispersibility and dispersion stability of pigments has been attempted previously and has included coating or adsorbing various surfactants onto the pigment. However, merely adsorbing or coating pigments has potentially several disadvantages. First, the amount of adsorbed surfactant on the pigment is dependent on the properties of the surrounding materials, such as the solvent for an ink. In such a situation, the surfactant may exist in an equilibrium between the surface of the pigment and the solvent. Solubility issues involving the surfactant and the solvent may also come into effect. Other considerations which can effect adsorbed surfactants are the concentration of the pigment, the type of surfactants used, and the temperature and pH of the composition or material containing the pigment. Any one or more of these properties can cause the removal of the surfactant from the surface of the pigment and create additional surfactant in the bulk liquid or medium. This, in turn, may negatively effect the dispersion stability and other properties of the bulk liquid such as foaming, surface tension, viscosity, film strength, film elasticity, and the like. In addition, the excess surfactant which may be found in conventional pigments may also be detrimental to plastic or film-forming properties, such as color, strength, elongation, elasticity, and the like.
One example of the use of adsorbing or coating a material onto a pigment is discussed in U.S. Pat. No. 5,200,164. This discloses the use of a compound containing at least one long chain alkenyl or alkyl group and at least one amine group as a treating agent to improve the dispersion characteristics of carbon black. While improved dispersibility does result, the treating agent used is not attached to the pigment and therefore the issues discussed above apply.
Thus, there is a desire in the art to improve the dispersibility and dispersion stability of pigments with respect to overcoming one or more of the above-described disadvantages. One approach to improving the dispersibility of pigments has been to attach at least one organic group to the pigment. The preparation of these types of pigments has been described in, for example, U.S. Pat. Nos. 5,851,280, 5,554,739, 6,042,643, and 5,837,045, as well as PCT Publication WO99/23174, all of which are incorporated in their entirety herein by reference. The organic group can be chosen in order to obtain stable dispersions in a variety of media. Aqueous and non-aqueous compositions comprising these modified pigments have also been described in U.S. Pat. Nos. 5,571,311, 5,713,988, 5,672,198, and 5,707,432, as well as PCT Publication WO99/51690, all of which are incorporated in their entirety herein by reference. While this approach does improve the dispersibility of pigments, there exists a need to obtain pigments which are dispersible in specific media, such as hydrocarbon solvents.
Pigment dispersibility is important when the pigment is used as the photothermal conversion material in a printing plate. Printing plates are used in several areas of image reproduction, including lithographic printing (also known as offset or planographic printing), flexographic printing, and gravure printing (also called intaglio or rotogravure). In general, the printing process involves the development of an image on the plate followed by exposure to an ink.
Lithographic printing plates are among the most widely used for making printed copies. Generally, an infrared or near-infrared laser-imageable lithographic printing plate includes at least the following layers: a grained-metal or polyester plate or sheet-like substrate and a radiation-absorptive layer coated thereon. Protective layers for the substrate or the surface of the coated plate may also be used which. When coated onto the substrate, this protective layer can also serve as an adhesion-promoting primer. Other layers may be used, for example, to improve adhesion between layers and durability of the printing plate.
In general, the radiation-absorptive layer comprises a photothermal conversion material capable of interacting with the imaging radiation and a polymeric resin or binder. In the imaging process, regions of the plate are selectively exposed to a laser output or other heat source capable of removing or chemically modifying the radiation-absorbent layer. Typically, heat sensitive lithographic printing plates are exposed to radiation having wavelengths of between 800 and 1200 nm. The laser output will define a pattern on the printing plate and either remove or chemically or physically modify only those regions of the radiation-absorptive layer which define the pattern. Afterwards, the printing plate can be further developed by subjecting it to a solvent capable of removing the exposed region(s), if any remains, which define the pattern or, if desired, the plate can be developed such that the non-exposed region(s) are removed. The details of the various conventional components and techniques for such printing plates are described in U.S. Pat. No. 5,493,971; U.S. Pat. No. 5,705,308; EP 0 803 771 A1; EP 0 770 494 A2; EP 0 770495 A1; as well as PCT Publications WO96/20429 and WO-98/31550 and the patents set forth therein, all of which are incorporated in their entirety by reference herein.
Several types of polymers have been used in the radiation-absorptive layer. Representative polymers include polyurethanes, poly(vinyl alcohol), polyacrylates, polystyrene, styrene-acrylate polymers, metal oxide polymers, epoxy resins, and phenolic polymers.
The photothermal conversion material can be either a pigment or a dye. For example, UV- and IR-active dyes have been disclosed in phenolic printing plate applications (see DBP 879205 and WO 97/39894). IR-absorptive pigments such as carbon black have also been shown to be useful in a lithographic printing plate (see, for example, WO 99/08157, WO 96/20429, WO 99/11458, and U.S. Pat. No. 6060218 in which carbon black is present in a phenolic polymer).
Pigments such as carbon black are broad band radiation absorbers and, as such, offer an improvement in performance over dyes. However, the effectiveness of pigments such as carbon black as a photothermal conversion material in a printing plate is dependent on the dispersibility of the pigment in the polymer. Thus, there is a need for printing plates comprising pigments such as carbon black with improved dispersibility in phenolic polymers used to produce printing plates.
Carbon blacks that have been modified to have carboxylate or sulfonate functionalities have also been disclosed in lithographic printing plates. For example, see WO 99/04974, WO 99/19143, WO 99/19144, WO 99/37482, and WO 99/37481. However, none of these disclose the use of modified carbon blacks comprising non-ionic groups in a printing plate application.
PCT Publication WO99/51690 discloses printing plates comprising at least one modified pigment product. These modified pigments comprise a pigment having attached at least one type of polymeric functionality. While such modified pigments offer improvements over the prior art, there remains a need for modified pigment products that provide good dispersibility and imaging performance in printing plate applications, as well as in applications including electrophoretic displays, inks, coatings, and plastics.